The invention pertains to the field of bidirectional communication of digital data over coaxial cable or other transmission media. More particularly, the invention pertains to the field of provision of multiple channels of digital data including interactive TV services, digital telephony, video teleconferencing, video on demand, internet access at 10 megabit/second or media data rates etc., all provided to home or business establishments over cable TV coax or combinations of coaxial cable, fiber optic links, microwave or satellite links or other wireless systems using synchronous CDMA system technology. These new services delivered over the CATV system will hereafter be called supplemental services.
In order to provide bidirectional digital data communication over a cable TV coaxial network to multiple subscribers with multiple digital services including data, digitized video and digitized audio, all made available over a single cable TV hybrid fiber coax (hereafter HFC) network (hereafter called CATV systems), several problems have to be solved. First, video applications such as video teleconferencing and video on demand movies require that a flexible amount of bandwidth be awarded to support the digitized video transmissions and that the amount of bandwidth awarded be guaranteed, i.e., the awarded bandwidth will be made available continuously without interruption until the video service is completed. These requirements force the use in the CATV environment of higher level protocols that have been developed or which are in the process of development for local area networks such as ATM or ISDN that are designed for delivery of digitized video, digitized audio and digital data over point to point LAN connections. Thus, the a major problem exists in adapting these point to point LAN protocols to the point-to-multipoint CATV environment.
Second, there is the problem of bandwidth availability and sharing the available bandwidth among a plurality of users who are physically distributed and all of whom have changing bandwidth needs over time. In CATV systems, typically only 6 mHz of bandwidth is available for communications of digital data downstream from the head end to the subscribers and another 6 mHz elsewhere on the spectrum of the CATV media is made available for upstream communications. These 6 mHz bands must be shared by all the users to carry data for all the services provided. Since video is a high bandwidth demand application which requires continuous availability, the available bandwidth must be fully utilized with maximum efficiency so as to provide bandwidth capacity to as many users as possible.
Third, there is the problem of noise and interference on the CATV system which could degrade the supplemental services to the point of unacceptability if techniques are not employed to lessen the affect of the noise on reception or otherwise deal with the problem. Cable TV media are high noise environments because of numerous reasons which are well known to those skilled in the art.
A fourth major problem, but related to the second problem, is synchronization of data transmission. Synchronization must be maintained between all remote units and the central unit for maximum efficiency use of the available bandwidth. If synchronization is not maintained, the number of users which can simultaneously share the available bandwidth is reduced.
In addition, there is the problem of eliminating the intersymbol interference and partial cross correlation between codes of code division multiple access (herafter CDMA) technology. CDMA technology is used in the genus of the invention to support multiple user with simultaneous pseudo point-to-point connections to the head end. CDMA is known in the prior art and has the advantage of not requiring the bandwidth allocation of frequency-division multiplexing nor the time synchronization of time-division multiplexing. However, it has the problems mention above which must be solved for a system acceptable to a user to exist. In addition, to maximize the number of users supported, frame synchronization is used in the invention and one of the problems solved by the inventors is how to achieve this frame synchronization in a physically distributed system of transmitters.
Of course all nonbaseband CDMA systems require two additional forms of synchronization as is well known in the art: first, there must be synchronization in phase and frequency between the transmitting carrier and the local oscillator in the receiver which feeds the demodulator. Also, as is known in the art all CDMA systems require clock recovery synchronization so that the pseudonoise code (hereafter PN code) sequence fed into the despreading circuitry is not only identical to the PN code fed into the spreading circuitry of the transmitter, but also exactly in phase therewith. Further, clock recovery is necessary in CDMA systems so as to know symbol boundaries so as to be able to correctly sample the baseband data stream exiting the despreading circuitry. How to achieve these forms of synchronization in a CATV CDMA system are another set of problems that must be solved.
Further, in CDMA systems with multiple transmitters which are physically distributed each using different spreading codes, it is possible for signals from different transmitters to arrive at the central unit at different power levels. This causes nonzero cross-correlations between the different codes, and gives rise to a problem known as the xe2x80x9cnear-farxe2x80x9d problem. To prevent high error rates from strong signals overwhelming weaker ones, the near-far problem must be solved.
An example of one attempt to transmit digital data bidirectionally over a CATV system is the technology disclosed in U.S. Pat. Nos. 4,912,721 and 5,235,619 assigned to Scientific Atlanta. In these systems, direct sequence spread spectrum technology is used to overcome the noise problem in CATV systems, but no support of multiple users by code division multiple access is sought. The Scientific Atlanta patents are single user direct-sequence spread spectrum systems, and there is no teaching of how to achieve frame synchronization which is necessary to maximize the number of users in a CDMA multiuser system. In the Scientific Atlanta patents, each user sends billing and request data to the head end and the head end sends data related to supplemental services other than the cable TV programming to the remote units at the sites of users during a timeslot assigned to that user. The transmission of data is accomplished on a one user per timeslot basis using a pseudonoise spreading code. No teaching of the use of orthogonal codes is present in U.S. Pat. No. 4,912,721. Therefore, only one user can use the available bandwidth devoted to the supplemental services at any particular time. Accordingly, for all users to share the available bandwidth, a time division multiplexing scheme must be used and the direct-sequence, spread spectrum technology is used only to overcome the noise and other impairments on the CATV channel.
CDMA cellular telephone systems are known. One example is U.S. Pat. No. 5,416,797 assigned to Qualcomm Inc. of San Diego, Calif. In this system multiple users of a cellular telephone systems are supported by a direct-sequence spread spectrum system which minimizes interference between users by providing orthogonal PN codes for spreading and controlling transmit timing such that the code frames from different users are aligned in time with each other at the receivers. Each cell site transmits a pilot carrier signal which is unique to that cell and which is used by the mobile units to obtain initial system synchronization and to provide time, frequency and phase tracking information. Each cell spreads the pilot signal using the same code, but cell uses a unique phase offset thereby allowing mobiles to achieve synchronization by searching all phase possibilities for the single code. The strongest pilot signal is tracked, and then the mobile unit acquires a synchronization channel which transmits system ID cna cell ID information as well as synchronization information which allows the long PN codes, interleaver frames and vocoders to be synchronized. When a call is initiated, a PN code is assigned for the call. During the call, pilot signals are continuously scanned to determined when another pilot signal becomes stronger. An outer PN code is used to spread all signals in the same cell to distinguish signals from other cells and multipath signals. A second inner code is used to distinguish between signals of different users in the same cell. The inner code is a maximal length PN code which is shifted in time for each user to provide discrimination among users. The patent states at Col. 11, lines 6-11, that because of the different propagation times between mobiles to different cells, it is not possible to satisfy a condition of time alignment required for the outer code Walsh function orthogonality for both cells at one time, so the outer code is used to distinguish between signals from different cells. Variable rate is used on the voice channel to prevent transmissions when there is no meaningful data to send. All cells in an area are synchronized to each other to an accuracy of 1 microsecond for easier handoff by tuning to GPS satellite transmissions which themselves are synchronized to Zulu time.
Cable networks typically involve a so-called head end or central unit from which video is transmitted to subscribers coupled to one or more main trunk lines from which extend numerous branch lines which may enter subscriber homes or which may couple to other branch lines. At each junction of a branch line to the trunk line or another branch line there is a directional coupler which is intended to direct transmissions from the head end to the subscribers in one direction and to direct transmissions from the subscribers back to the head end without leaking energy intended for transmission to the head end into branch lines coupled to other subscribers. In order to send digital data over video coax, a modem is necessary at both the head end and at all the subscriber locations to modulate digital data onto the coax as RF signals, and to receive RF signals carrying digital data and derive the digital data therefrom. Because RF signals are propagating along the cables, and because the couplers are not perfect, reflections occur at the directional couplers that cause noise and interference. This is because the reflections are frequently of the opposite polarity depending upon the impedance mismatch and the distances involved. These reflections are therefore sometimes additive and sometimes subtractive, thereby resulting in random variations in the amplitudes of the RF signals. These random variations make discrimination during the demodulation process to derive the digital data more difficult.
Further, because the subscribers are at physically different distances from the head end, the signals from each subscriber""s modem arrive at the head end at different times because of different propagation delays. Because digital data is transmitted in frames and because all subscribers must be synchronized to the same frame timing, these different propagation delays for each subscriber cause problems in synchronizing data.
In the typical interactive system, there are bidirectional amplifiers. Each amplifier has two channels, one of which amplifies signals in a high frequency range from 45-750 mHz for transmission of data from the head end to subscribers, and the other of which amplifies signals in a low frequency range from 5-42 mHz for transmission of data from the subscribers to the head end. When the operating frequency is close to the edges of either the high or low band, group delay distortion can result in another impairment to noise free bidirectional communication.
Other forms of linear impairment result from the fact that the bandpass filters in the system do not have perfectly flat amplitude characteristics for their transfer functions across the entire passband, especially at the corner frequencies.
Other forms of impairments are so-called additive impairments resulting from taps on the system which are not properly terminated. These taps act as antennas and pick up broadcast signals from FM stations, CB radios, HF communication etc. Other sources of noise are impulse signals resulting from arcing in electrical appliances near taps. Arcing generates white noise type RF energy that is splattered across all bands and is radiated much like lightening. In addition, the amplifiers in the interactive system can sometimes break into oscillation thereby creating noise. Further the horizontal and vertical oscillators in television sets in the subscriber""s household also can radiate RF interference signals. These sources of interference are a major contributor to noise in the system.
The collection of impairments described above are referred to as linear impairments because the system has a linear transfer function so that in the Fourier domain, all the impairment effects are additive.
In addition to the linear impairments, there also exist nonlinear impairments such as second and third order harmonics generated by the nonlinear junctions of transistors in the amplifiers in the system. When multiple sinusoidal signals arrive at the amplifiers, beat frequencies are generated in addition to the harmonics of the arriving signals because the nonlinear junctions act like mixers.
Other forms of nonlinear impairment are hum from saturation of magnetic cores of transformers in the amplifiers resulting from 60 volt, 60 Hertz AC square wave pulses sent to the amplifiers over the coax to supply the amplifiers with power. The amplifiers have rectifiers that rectify this square wave signal to derive power therefrom and this process generates noise in the form of hum. This hum is caused by amplitude modulation of the power supply signal resulting from the placement of the operating point on the hysteresis curve of the rectifier transformers.
Interactive systems typically involve in excess of one hundred different channels on which separate digital data streams can flow in addition to the separate channels on which the video signals are provided for normal cable TV service. To send digital data as RF signals, very complex constellations of separate amplitude and phase combinations are used to encode the digital characters being transmitted. Because of the large number of data points, the differences in phase and amplitude between the different points are not large. Therefore, the impairments described above can cause errors by causing misinterpretation by demodulators of what characters were actually sent.
There is only one conductive path between the head end and the subscribers that must be shared by all the subscribers to send and receive digital data. One approach that has been tried in prior art interactive cable TV systems is time division multiple access sharing (TDMA) with quadrature phase shift keying (QPSK) modulation schemes. In a TDMA system, each subscriber gets a short assigned time slot in which to transmit data to the head end. The TDMA approach has shortcomings in that it causes difficulty and complexity in achieving xe2x80x9calignmentxe2x80x9d. Alignment refers to the proper timing of each subscriber""s transmissions so that they arrive during the appropriate time slot at the head end despite the fact that the signals from each subscriber have different propagation times. Alignment in TDMA multiple access schemes is critical, and achieving it is difficult.
Another difficulty with the TDMA approach is the high susceptibility of QPSK modulation to narrowband interference. Narrowband interference results when a signal like Voice of America or a harmonic which has a bandwidth similar to the bandwidth of the channels upon which digital data is being transmitted enters the transmission media. Typically this happens at a tap which is not properly terminated. Narrowband interference adversely affects the receiver circuits and the alignment circuits that achieve synchronization of all the sources.
The use of TDMA schemes also complicates the alignment problem in the presence of narrowband interference and other noise because there is such a short time for synchronization. Synchronization of the head end to all the subscribers is necessary so that the clock signal and other reference signals such as the carrier can be recovered from the transmitted signals without being separately transmitted. These recovered signals are critical to proper operation of the system in demodulating the transmitted data. Narrowband interference also adversely affects the decision making circuits that decipher which characters have been transmitted thereby increasing the error rate. Increased error rate requires more bandwidth to be consumed in retransmitting data and wastes processing power in detecting and correcting errors and retransmitting data with errors that are beyond the correction range of the ECC bits transmitted with the data.
Also, because in TDMA schemes the timeslots are very short, TDMA systems are susceptible to loss of data caused by long bursts of noise. Typical bursts of impulse noise are quite long relative to the length of the TDMA timeslots. Long bursts often wipe out entire groups of timeslots thereby requiring retransmission of all the data in these timeslots. Modulation and multiple access schemes that spread out the energy of transmitted data over longer times, such as FDMA schemes, are less susceptible to this type of burst noise. However, FDMA schemes have other drawbacks.
QPSK modulation used in TDMA schemes is also not the most efficient modulation scheme in terms of spectral efficiency, i.e., speed of transmission in bits/second/Hertz over a given bandwidth. However, QPSK modulation is used because it has a sufficiently large distance between points in the constellation to maintain relatively good noise immunity. Noise immunity refers to the ability of a system to achieve a certain bit error rate that is deemed to be tolerable for a given signal to noise ratio. If the signal to noise ratio decreases for a given noise immunity characteristic, the bit error rate will rise.
Typically, channel bandwidths are 6 mHz, and spectral efficiency of 27 megabits/second/Hertz is desired. Therefore, a data transfer rate of 162xc3x971012 is achievable over one channel at this typical spectral efficiency and bandwidth. However spectral efficiency for QPSK modulation is on the order of 1 bits/second/Hertz or less so this modulation scheme is too slow for high traffic volume applications such as video on demand, video teleconferencing etc.
Another approach that has been tried in the prior art is frequency division multiple access (FDMA). In FDMA, each subscriber transmits data on a different carrier frequency as opposed to at a different time in TDMA. FDMA schemes suffer from different drawbacks than TDMA schemes. Because there are multiple carriers in FDMA schemes, there are more intermodulation products in the received signals. These intermodulation products can coherently add to each other thereby causing peaks in amplitude which are far greater than the average signal. This causes difficulty in designing analog circuits with adequate ranges of linearity to handle these signals without clipping the peaks. These peaks often penetrate into the nonlinear or saturation range of operation of analog circuits in the system exposed to these signals thereby introducing nonlinearities. Even without these nonlinearities, the intermodulation components tend to cause crosstalk between adjacent channels which increases the error rate.
FDMA schemes are also sensitive to narrowband interference signals that suddenly appear causing unanticipated xe2x80x9cjammingxe2x80x9d of one or more channels thereby causing long bursts of errors. FDMA channels are narrow, and claims have been made that because of this fact narrowband interference can be avoided. However, narrowband interference is dynamic, because there are multiple sources thereof at different frequencies. As a result, narrowband interference signals can suddenly xe2x80x9cpop upxe2x80x9d when a subscriber turns on his or her TV or when Voice of America starts broadcasting. This sudden pop-up interference can jam a channel thereby causing error bursts.
Therefore, a need has arisen for a method and apparatus that can support interactive digital systems that eliminates some of these drawbacks.
According to the teachings of the invention, there is provided a code division multiplexing multiple access (CDMA) scheme using orthogonal codes to encode multiple channels of digital data for simultaneous transmission over a cable television media which is also carrying frequency division multiplexed cable television programming. Further, in the preferred embodiment, alignment of multiple subscriber remote units at diverse locations on the cable television media to the same frame alignment reference is used to substantially reduce crosstalk between adjacent codes and allow multiple users to simultaneously share the same cable TV media for auxiliary services other than cable TV programming delivery. The ranging process described herein is useful for any digital communication system which delivers data from physically distributed transmitters to a central location in frames, but in the context of a CDMA system on a cable TV plant, it provides for synchronous CDMA which greatly increases system payload capacity. The use of synchronous CDMA coupled with frequency division multiplexing of upstream and downstream data on different frequencies than the cable TV programming provides a system whereby the entire bandwidth devoted to the digital auxiliary services may be simultaneously shared by multiple users who share a plurality of channels. Any of the known ways of achieving frame alignment may be used to achieve synchronous code division multiple access data transmission. In the preferred embodiment, frame alignment is achieved by alignment of timing signals transmitted by remote units to guardbands or gaps between frames.
One inventive concept disclosed herein is to achieve high noise immunity by spreading the energy of the transmitted data out over time during transmission, and then compressing the energy again at the receiver to recover the data. Spreading the energy of the transmitted data out over time reduces susceptibility to burst errors and impulse noise. In addition to this spreading concept, the spectral efficiency of the system is enhanced by transmitting multiple separate channels of data over the same media without interference by using separate orthogonal codes to encode the data of each channel so that no interference results when all channels are simultaneously transmitted so long as proper frame alignment is maintained. In this way, the spectral efficiency, i.e., a measure of the amount of data that can be sent from one place to another over a given bandwidth, is enhanced without degradation of the data by crosstalk interference. The orthogonality of the codes used for each data stream minimizes crosstalk between data streams where the system is properly aligned, i.e., synchronized, and, using cyclic, orthogonal codes further enhances noise abatement by providing the ability to perform equalization. Equalization, as that term is used herein, refers to the process of determining the amount of crosstalk between adjacent codes resulting from minor errors of frame timing alignment and then generating signals which can be used to negate the crosstalk. In the preferred embodiment, the orthogonal codes are cyclic codes.
In some species within the genus of the invention, code diversity is used to achieve further noise immunity. It has been found that some orthogonal codes are less immune to narrow band interference and other sources of noise than others. To avoid using such codes to spread the data from the same channel or timeslot all the time, code hopping is used in these preferred species of the inventive genus. Code diversity is achieved in several different ways, but, in the preferred embodiment, each transmitter uses a code shuffler circuit and each receiver uses a code deshuffler circuit. All shuffler and deshuffler circuits receive the same seed and generate the same sequence of pseudorandom numbers therefrom. These pseudorandom numbers are used to generate read pointers to a framer memory and write pointers to a buffer memory. The framer memory is where the information vectors or symbols are stored, and the read pointers generated by the shuffler circuits are used to read the timeslot data, i.e., symbol/information vector elements out in pseudorandom fashion and store them in a buffer in accordance with the write pointers generated by the code hopping shuffler circuit. The information vector elements thus stored in the buffer are used to do the matrix multiplication required by the code division multiplexing scheme. Alternatively, the symbol elements may be read out sequentially from the framer memory and stored pseudorandomly in the buffer.
The effect of this synchronous CDMA scheme is to xe2x80x9cwhitenxe2x80x9d the noise sources such that no matter how complex the noise signals, the noise can be effectively managed using conventional error detection and correction bits. In other words, the digital data providing the interactive or bidirectional data communication is sent using a CDMA scheme, but for purposes of synchronization, the CDMA scheme is mixed with a TDMA scheme. More precisely, a guardband free of data is added to the CDMA signal. Digital data is transmitted in frames, each frame comprising 3 data symbols and a guardband. The guardband is used for non-data usage such as ranging, alignment and equalization.
The synchronous CDMA modulation scheme disclosed herein may be used with any shared transmission media and with any apparatus or method that can get all remote units synchronized to the frame timing of the central unit including the ranging/alignment scheme disclosed herein. Other possible methods of synchronizing to the same frame timing are for all remote units and the central unit to receive the same timing reference signals from some source such as internal atomic clocks or from an external source such as a Global Positioning System satellite from which all remote units and the central unit are effectively equidistant.
Likewise, the ranging/alignment scheme disclosed herein is useful for any other modulation scheme which transmits digital data in frames, requires frame synchronization and can insert a guardband between the frames.
Some species within the inventive genus use M-ary modulation code division multiplexing. Each remote unit receives a time division multiplexed stream of digital data. Each timeslot contains 9 bits of data. Each 9 bits is stored in a framer memory, and is divided into three tribits, each having 3 bits during readout of the memory. Each of the three symbols transmitted each frame is comprised of 144 of these tribits, one for each timeslot or channel. These tribits are encoded with a 4th bit prior to spreading by the code division multiplexing operation. The 4th bit is added to each tribit based upon the three bits of the tribit and based upon the previous state for this timeslot""s data during the last frame. This 4th bit adds sufficient redundancy to enable a Viterbi Decoder in the central unit receiver to make a more error free determination of what data was actually sent in the presence of noise. The 4th bit also maps each tribit to a 16 point QAM (quadrature amplitude modulation) constellation by using the first two bits to represent the inphase or I axis amplitude and the last two bits to represent the quadrature or Q axis amplitude. Thus, M-ary modulation is used to achieve greater spectral efficiency.
With the system described herein, full 10 megabit/second traffic volume per each 6 MHz channel can be achieved in both the upstream and downstream direction over HFC. Unlike conventional CDMA, SCDMA transmission from transmitters like that shown in FIG. 28A stays within 6 MHz bands that do not interfere with or effect other adjacent channels. SCDMA has a number of other advantages over pure FDMA and TDMA systems in terms of capacity, scalability and bandwidth allocation. Standard IS-95 asynchronous Code Division Multiple Access spread spectrum systems are hindered by the capacity constraints of the 5-40 MHz upstream channel and the presence of a large amount of noise, and they often require 30 MHz wide channels which creates channel interference problems with neighboring services in the HFC spectrum. The biggest problem with asynchronous CDMA systems is self-generated noise because the RUs are not aligned with each other thereby losing orthogonality and creating a high degree of mutual interference. The higher self-generated noise raises the noise floor and reduces the capacity. SCDMA system insure that the RUs are in frame synchronization with each other and using orthogonal codes to minimize mutual interference as data is sent upstream. Preferably, SCDMA transmitters are also used to send data downstream. With the system described herein, multiple streams of digital data, each having a 64 kbps throughput can be simultaneously sent over a 6 MHz channel with a total 10 Mbps throughput. Each data stream is Trellis encoded, interleaved and spread over the entire 6 MHz using its own individual spreading code. Use of forward error correction and interleaving increases noise immunity to impulse noise, narrowband interference and Gaussian noise. The Trellis coding adds 4.8 dB coding gain, and interleaving enables withstanding long duration impulse noise of up to 100 microseconds without incurring errors. Use of spread spectrum technology adds another 22 dB processing gain. The combination of techniques yield a total 27 dB interference rejection allowing the system to operate in negative Carrier to Noise Plus Interference Ratio. The SCDMA transmitters are combined with TDMA payload data input streams which makes the system extremely scalable.
The high capacity of the SCDMA system disclosed herein is made possible by orthogonality which is made possible by the orthogonality of the spreading codes which is a result of the ranging process and the equalization process. The ranging process assures frame synchronization such that all codes arrive from distributed RUs arrive at the CU at the same time. The ranging process is carried out periodically to account for cable expansion/contraction with changing temperature, but the process is transparent to payload traffic in that it does not slow it down, stop it or cause errors. Re-ranging occurs upons certain error conditions and upon disconnect from the network and each powerup.
Equalization is achieved by measuring the channel response from each user to the CU and adjusting a precoder at the RU transmitter to xe2x80x9cinvert the channelxe2x80x9d, i.e., predistort the transmitted signal such that it arrives undistorted at the CU. Power alignment by each RU such that each RU transmission reaches the CU at approximately the same power level also helps to minimize mutual interference.
Dynamic bandwidth allocation allows as many 64 kbps streams or channels as necessary to be allocated to a particular service so that high demand applications such as video teleconferencing or high speed internet access can be supported simultaneously with low demand applications like telephony over the same HFC link. Bandwidth allocation is managed at the CU through an activity status table in each RU and the CU that indicates the status of each timeslot and code assignments. The CU updates the RU tables by downstream messages. Bandwidth can be guaranteed upon request while other services with more bursty traffic may contend for the remainder of the total 10 Mbps payload.
The advantages over TDMA systems include less need for fast acquisition and correspondingly lower sensitivity to narrowband interference. Further, below a certain SNR, TDMA systems may fail altogether. Contention for certain channels and contention affecting adjacent can cause amplifier overload in TDMA systems and can cause severe throughput and performance problems. FDMA systems where each user gets a narrow upstream frequency slice is very susceptible to narrowband noise which can wipe out an entire channel. FDMA systems often try to avoid this problem with frequency reallocation. This complicates and raises the cost of the system by requiring more intelligence. Throughput is also adversely affected as nothing is sent while frequencies are reallocated. Guardbands between channels waste bandwidth, and frequency misalignment degrades FDMA systems.
Any method or apparatus that uses these inventive concepts is within the teachings of the invention and is deemed to be equivalent to the apparatus and methods described herein.